Production of metallic magnesium



Patented Sept. 18, 1945 PRODUCTION OF METALLIC MAGNESIUM Joseph D. Hanawalt and T Melville Hess, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Michigan No Drawing. Application March 2, 1942, Serial No. 433,010

1 Claim.

This invention relates to the Production of substantially iron-free metallic magnesium by the electrolysis of fused magnesium chloride.

The production of metallic magnesium by the electrolysis of fused baths essentially comprising magnesium chloride is almost invariably carried out in vessels constructed of steel or other ferrous metal. As a. result, the metallic magnesium formed usually contains as an impurity iron in a proportion approaching its solubility limit in magnesium, i. e. 0.02 to 0.03 per cent by weight. While the presence of this small proportion of iron is not disadvantageous in many of the commercial applications of magnesium, it is sometimes desirable, as in the production of magnesium-base alloys of extremely high corrosion-resistance, to utilize substantially ironfree magnesium,.i. e. metal containing less than about 0.002 per cent by weight of iron. Preparation of magnesium containing iron only to to this low degree has been accomplished heretofore, but only by specialized treatments which must be applied to the metal subsequent to its electrolytic production and which require considerable apparatus for their effectuation.

The principal object of the present invention, then, is to provide an improved electrolytic process for making magnesium wherein substantial y iron-free metal is produced directly during the course of the electrolysis, whereby the necessity of further purification treatment is eliminated.

According to the invention, the production of substantially iron-free magnesium, in processes wherein a fused bath essentially comprising has a low rate of volatilization at operating temperatures. Metallic boron, and particularly boric acid and its alkaliand alkaline-earthmetal salts, e. g. borax, magnesium borate, and calcium borate, are especially suitable; other boron-containing materials may, however, be employed.

In practicing the invention, the production of metallic magnesium is, except for the addition of the boron material, carried out entirely in accordance with known electrolysis procedures.

The electrolytic cell bath is usually either fused magnesium chloride or a fused metal halide mixture consisting largely of magnesium chloride together with one or more other salts, such as sodium chloride, added to increase the fluidity or for other purposes; operating temperatures,

current densities. etc. are those conventionally used. The boron-containing material may be introduced into the electrolysis bath in any desired manner. However, since there is ordinarily a slow loss of boron material from the cell bath, probably by volatilization, it is preferable to add the boron-containing substance in small portions at frequent intervals to replenish that lost. A particularly convenient procedure is to feed the boron material into the cell vessel together with the magnesium chloride which is customarily added thereto in regular manner to make up for that consumed in the electrolysis.

The proportion of boron-containing material added may vary within considerable limits. In general, however, a quantity corresponding to between about 0.002 per cent and about 0.03 per cent by weight of boron relative to the magnesium chloride being added to the cell is preferred, since with quantities of boron much below 0.002 per cent, the magnesium produced may not be satisfactorily free of iron, whereas with quantities much above 0.03 per cent, the emciency of the electrolytic cell tends to be decreased .to an extent such that economic operation may not be practicable under all conditions.

It has been found that the iron-removing effect of boron compounds as described is interfered with to some degree by salts of manganese. It is desirable, therefore, that the magnesium chloride used in the electrolysis according to the invention be substantially free of such material.

A particularly convenient form of the new process is possible when the magnesium chloride used in the electrolysis is derived from sea water or other magnesium-containing natural brine in which inorganic boron compounds are also present. In this case the process by which msgnesium chloride is recovered from the brine may be controlled so that the magnesium chloride, instead of being substantially boron-free, as is ordinarily the case, retains a quantity of the boron compounds from the brine sufllcient to fall within the limits given.

We claim:

In a process of makingmetallic magnesium by the electrolysis in a ferrous metal vessel of a fused salt. bath essentially comprising magnesium chloride and being substantially free of manganese salts and adapted to yield magne slum in electrolysis, the method of insuring production of substantially iron-tree magnesium which comprises introducing into the vessel together with the magnesium chloride 0. compound selected from the'class consisting of borlc acid and its alkaliand alkaline-earth-metal salts, in a proportion corresponding to between about 0.002 per cent and about 0.03 by weight of boron relative to the magnesium chloride introduced.

JOSEPH D. HANAWALT. '1' mvnu: mesa 

